Assert the builder can reach the broker, not merely that it is running

A builder that cannot connect sits there, and every outward sign — container
up, credential on disk — says it is working. The failure surfaced five minutes
later as nothing consuming the build queue, which names no cause at all.

test/integration/mesh.test.ts

@@ -676,13 +676,25 @@ test("the builder is a module the mesh assigns, with a credential the mesh deliv
     `the builder was assigned and is not running:\n${running}\n` +
     `${(await on("anchor", `tail -30 /var/log/mesh-host.log`)).out}`);
 
+  // Running is not connected. A builder that cannot reach the broker sits there, and every
+  // outward sign — the container is up, the credential is on disk — says it is working.
+  await new Promise((r) => setTimeout(r, 5000));
+  const said = await on("anchor", `docker logs mesh-builder 2>&1 | tail -20`);
+  assert.doesNotMatch(said.out, /cannot reach the broker/,
+    `the builder is running and cannot reach the broker:\n${said.out}`);
+
   // The credential arrived, is readable only by the machine, and is the scoped account rather
   // than the broker's own.
   assert.match(await must("anchor", `stat -c %a /var/lib/mesh/builder/broker`), /^600/);
   const credential = await must("anchor", `cat /var/lib/mesh/builder/broker`);
-  assert.match(credential, /^amqps:\/\/lab-builder:/,
+  assert.match(credential, /"url":"amqps:\/\/lab-builder:/,
     "the builder is using an account that is not its own");
   assert.doesNotMatch(credential, /guest:guest/, "the builder holds the broker's own account");
+  // And what to check the broker against. A mesh's broker presents a certificate of the mesh's
+  // own, so a URL alone reaches only a broker some public authority vouches for — which is no
+  // mesh broker at all, and fails at TLS with an error about an unknown authority.
+  assert.match(credential, /"fingerprint":"[0-9a-f]{64}"/,
+    `the builder was given nothing to verify the broker with:\n${credential}`);
 
   // And it works: the mesh asks this builder to build something, and it does. Answering is the
   // only proof that the delivered credential authenticates — a container that is up with a